Anti-apoptotic proteins Bcl-2 and Bcl-xL are overexpressed in many cancers and contribute to tumor initiation, progression and resistance to therapy. Molecular modulation of Bcl-2/Bcl-xL represents a promising strategy for overcoming the resistance to apoptosis induced by current cancer therapy. The potent, sequence-specific gene silencing by small interfering RNA (siRNA) has become a powerful tool in cancer research and holds significant potential as novel molecular therapy for cancer. However, delivering the siRNA-based therapeutics efficiently and specifically to tumor and its metastases remains a great challenge. We have developed a tumor-specific, ligand-targeting, self-assembled DNA-nanovector system which shows promising efficiency and specificity in targeted delivery of various genes and anti-sense oligonucleotides to human cancer, with limited effect on normal tissues (US Patent No. 6,749,863). We have also designed siRNAs for human Bcl-2 and Bcl-xL that can potently knock-down Bcl-2/Bcl-xL leading to extensive cancer cell death (US Patent pending). We propose to use our patented nanovector system to develop siRNA-based therapeutics for tumor- targeted silencing of Bcl-2/Bcl-xL. We will test two inter-related hypotheses: (1) Tumor-targeted delivery of siRNA will efficiently silence Bcl-2/Bcl-xL, and induce apoptosis in human cancer cells that depend on Bcl- 2/Bcl-xL for survival;(2) Knock-down of the anti-apoptotic Bcl-2/Bcl-xL in turn will overcome resistance and restore sensitivity of cancer cells to chemo/radiotherapy. Our long-term goal is to develop the tumor- targeting siRNA-nanovectors as novel molecular therapy targeting Bcl-2/Bcl-xL for human cancers with Bcl- 2/Bcl-xL over-expression. To test our hypothesis, we propose to carry out three SPECIFIC AIMS: AIM 1: To prepare and optimize the siRNA-nanovectors for efficient siRNA delivery to human tumors in vitro and in vivo;AIM 2: To investigate in vitro anti-tumor activities and the mechanism of action of siRNA-nanovectors in combination with chemo/radiotherapy;AIM 3: To investigate the in vivo therapeutic potential of Bcl-2/Bcl-xL siRNA-nanovectors in nude mouse xenograft models of human cancers with high levels of Bcl-2/Bcl-xL Combining siRNA-based Bcl-2/Bcl-xL molecular therapy with conventional therapy would improve the efficacy and overcome the resistance to current cancer treatment, especially for tumor metastasis, in which Bcl-2/Bcl-xL protein is overexpressed and for which conventional therapy is not very effective. Successfully carried out, our studies will provide proof-of-concept that siRNA can be delivered by the self-assembled nanovectors for tumor-targeted silencing of the genes critical for cancer progression and resistance.